1. Field of the Invention
The present invention relates to a method of developing an electrophotographic film using a liquid developer, and an apparatus which may suitable be employed to carry out said method.
2. Description of the Related Art
One type of photographic apparatus has heretofore been known which is capable of recording an image on a predetermined frame of a photographic film and of projecting or copying the recorded image. This type of apparatus is disclosed in, for example, U.S. Pat. Nos. 3,528,355, 3,697,173, 3,964,828, 3,972,610 and 4,461,561.
A processing head is disposed in such photographic apparatus to subject an electrophotographic film to various kinds of processing such as charging/exposure and development and such processing head is known from the specifications of U.S. Pat. No. 4,600,291 and U.S. patent application Ser. No. 696,590 filed Jan. 31, 1985.
One type of electrophotographic film is formed by successively coating an electrically conductive layer and a photosensitive layer on a carrier. Such electrophotographic film is electrically charged and then exposed to form an electrostatic latent image on the photosensitive layer, and the film is then developed to make the latent image visible. For development of this type of electrophotographic film, it is general practice to employ a liquid developer which is formed by dispersing into a solvent tonner particles which are charged opposite in polarity to the electrophotographic film that is electrically charged In development of the film, the above-described liquid developer is supplied onto the electrostatic latent image to make it visible.
The processing head of the type described above is provided with a developing section having an electrode which is generally disposed so as to face the photosensitive layer side of an electrophotographic film. A portion of the film which has an electrostatic latent image formed thereon is positioned at the developing section, and the above-described liquid developer is supplied to a gap defined between the photosensitive layer side of the film and the developing electrode, thereby developing the film. The presence of the developing electrode enables obtaining of an image having no edge effect In addition, the application of a bias voltage to the developing electrode prevents fogging of the background of the image, the bias voltage having the same polarity as that of the charged photosensitive layer.
The photosensitive layer having an electrostatic latent image formed thereon has a relatively low potential at a portion thereof which will result in a solid half-tone portion of a finished image For this reason, the toner electrostatically adheres to this portion at a relatively low speed, which gives rise to a problem in a developing system wherein a liquid developer is allowed to flow from the upper side to the lower side of an electrophotographic film positioned at the developing section. More specifically, in such developing system the speed of flow of the liquid developer is higher than the speed at which toner particles are moved toward the solid half-tone portion by the effect of electrostatic field, so that a less amount of toner adheres to a region of the solid half-tone portion which is positioned on the upstream side as viewed in the direction of flow of the developer, resulting disadvantageously in an image having a nonuniform density at the solid half-tone portion.
A low-contrast thin-line portion of the electrostatic latent image also has a relatively low potential, that is, a relatively weak electrostatic force for attracting toner particles. Therefore, the speed at which toner particles adhere to such thin-line portion is low as compared with the speed of flow of the liquid developer, and this leads to occurrence of the phenomenon that no toner particles adhere to the thin-line portion of the latent image. Such tendency is particularly noticeable in a thin-line pattern which extends perpendicularly to the direction of flow of the liquid developer. Thus, the conventional developing method using a liquid developer involves the problem that a finished image has an undesirably low density at a portion including a thin-line pattern such as a character.